Biogas is a mixture of primarily methane and carbon dioxide gases. It is considered to be a low-grade natural gas, as it contains approximately from 50-65% methane. In comparison, natural gas contains approximately from 90-95% methane. Biogas is seen as a cost-efficient or low-cost fuel, as it is a renewable fuel.
Biogas is produced when bacteria convert organic matter to methane. Organic matter is the food source for methane-producing bacteria. For farm-based biogas production, the primary organic matter source is manure. Although biogas can be produced using manure as the only organic source, research has shown that gas production can be greatly increased by adding additional substrates. For example, the most common substrate is derived from energy crops such as corn silage. However, the digestion of fiber has been shown to be a limiting factor in the use of such energy crops, such as silage.
The plant cell wall is a complex structure consisting of different polysaccharides, the major components being cellulose, hemicelluloses and pectins. These polysaccharides may be cross-linked, or linked to lignin by phenolic acid groups such as ferulic acid. Ferulic acid may play a role in the control of cell wall growth in the plant and ferulic acid cross-linking within the cell wall is believed to restrict cell wall digestion by microorganisms (Fry et al., (1983) Planta 157: 111-123; and Borneman et al., (1990) Appl. Microbial. Biotechnol. 33: 345-351). Some microorganisms are known to exhibit ferulic acid esterase activity (ferulate esterase) and thereby facilitate the breakdown of plant cell walls and fiber digestion (U.S. Pat. No. 6,143,543). U.S. patent application Ser. No. 11/217,764 demonstrates the importance of methods and compositions to decrease the resistance of the plant cell wall to digestion for improvements in the animal production industry through the use of silage inoculants, forage additives and other amendments that improve the digestibility of feed.
The feed industry has used various treatments for ensiled feed or other animal feed with fiber degrading enzymes, including, for example: fungi, including various mold sources, to improve feed digestibility; Saccharomyces cerevisiae yeast strains fed directly to cattle to improve fiber digestion (Erasmus et al., (1992) J. Dairy Sci. 75: 3056-3065; and Wohlt et al., (1998) J. Dairy Sci. 81: 1345-1352); direct microbial feeds, including a species of Lactobacillus (WO-A-93/3786; and U.S. Pat. No. 6,699,514); feeding a diet inherently possessing good digestibility characteristics, including for example, brown midrib corn silage (Oba and Allen, (1999) J. Dairy Sci. 82: 135-142), or alternatively, highly digestible corn hybrids; and technologies incorporating fungal gene(s) for the production of ferulate esterase into plant tissue for subsequent expression, resulting in improvements in fiber digestibility (WO 02/68666).
The successful enhancement of fiber digestion in the rumen of an animal results in the animal getting more from its feed. As a result, the animal demonstrates increased milk yield, in dairy cows, and beef production in forage fed animals. Accordingly, farmers either tolerate a lower level of feed digestibility from silage, and therefore productivity, or use inoculants, forage additives or other feed additives to improve digestibility of feed.
There are significant intrinsic similarities between the need for improved digestion of silage in the rumen of an animal and anaerobic biogas digestion. As a result, the biogas industry has extensively considered methods for improving the production of biogas from energy crops such as corn silage.
The utilization of energy crops for stimulation of biogas production yields similar limitations as those observed in feed digestion. Namely, the fiber fraction of crops is difficult to degrade, such that the fiber portion of the biomass is the first and rate-limiting step in the production of biogas from energy crops, including for example, corn.
To overcome the limitation of fiber degradation of organic sources, namely silage such as corn, a cocktail of fiber-degrading enzymes may be applied directly to the anaerobic digester or to the silage prior to delivery to the digester. In this instance, the applied enzymes work to partially degrade the fiber fractions, releasing simple substrates such as sugars and carbohydrates to provide additional energy to the methane-producing bacteria. The result is increased fiber degradability. However, neither the typical fermentor nor silage provides ideal conditions for the actions of these enzymes. Therefore, fiber degradation will continue to proceed at a relatively slow rate.
An additional means to overcome the limitation of fiber degradation in organic silage sources is to increase its surface area. The degradation of particulate fiber material is dependant upon the availability of surface area for exogenous enzyme or bacterial attachment. The smaller the particle size of the matter results in a greater surface area. As a result of such increased surface area, the rate and the extent of digestion of the fiber fraction are both faster and greater. Silage from energy crops has a large particle size due to the necessity for packing such silage to exclude air. Additionally, reduced particle size affects the function of an animal's rumen, such that inadequate effective fiber is available to stimulate the rumen. Accordingly, the optimal conditions for silage management and animal feeding are not the same as those needed for optimal degradation of fiber to produce methane.
Processing silage in order to reduce its particle size can be used to further increase the digestion of the fiber fraction in methane generators. However, the technique requires specialized equipment, additional handling and energy inputs to improve the production of methane. As a result, such processes are not cost-effective for biogas production.
Accordingly, it is an objective of the claimed invention to provide methods for enhancing the production of biogas through the use of silage inoculants to improve the degradation of the fiber portion of the organic materials.
A further object of the invention is a method for treating pre-ensiled plant material to enhance the production of biogas.
A further object of the invention is a method for enhancing biogas production yielding an increased rate and increased extent of digestion of fiber fractions.
A further object of the invention includes compositions for use as silage inoculants.